In Java(tm), null is not a keyword, but a special literal of the null type. It can be cast to any reference type, but not to any primitive type such as int or boolean. The null literal doesn't necessarily have value zero.
Add a jump rope to the motion and you will be skipping rope. The skipping moves won't bulk you up, but they can tone the muscles in your thighs, calves and butt -- your quadriceps, hamstrings, gluteals, iliopsoas, gastrocnemius and tibialis anterior.
Performing jumping jacks works the arms and legs while increasing your heart rate. Stand with your feet together and arms at your sides. Jump up while bringing your arms out to the sides and straight above your head. Land with your feet slightly wider than shoulder-width apart.